A brushless direct-current (BLDC) motor may be classified into a core type or a radial gap type and a coreless type or an axial gap type, each having a generally cup-shaped cylindrical structure, according to whether or not a stator core exists.
A BLDC motor of a core type structure may be classified into an inner magnet type including a cylindrical stator where coils are wound on a plurality of protrusions formed on the inner circumferential portion thereof in order to form an electronic magnet structure, and a rotor formed of a cylindrical permanent magnet, and an outer magnet type including a stator where coils are wound up and down on a plurality of protrusions formed on the outer circumferential portion thereof, and a rotor formed of a cylindrical permanent magnet on the outer portion of which multiple poles are magnetized.
The magnetic circuit in the above-described core type BLDC motor has a symmetrical structure in the radial direction around the rotational shaft. Accordingly, the core type BLDC motor has less axial vibration noise, and is appropriate for low-speed rotation. Also, since a portion occupied by an air gap with respect to the direction of the magnetic path is extremely small, a high magnetic flux density may be obtained even if a low performance magnet is used or the amount of magnets is reduced. As a result, a big torque and a high efficiency may be obtained.
However, such a core, that is, a yoke structure may cause a big material loss of a yoke when manufacturing a stator. In addition, a special expensive dedicated winding machine should be used in order to wind coils on the yoke due to a complex structure of the yoke when mass-producing. In addition, a mold investment cost is high at a time of manufacturing a stator, to thus cause a high facility investment cost, in the case of the core type BLDC motor.
The core type alternating-current (AC) motor or the core type BLDC motor, especially, the core motor of the radial gap type, has high competitiveness, since coils can be wound on split type cores with high efficiency using a general purpose winding machine which is cheaper than a special-purpose expensive dedicated winding machine in the case that the stator core is configured into a complete split type. On the contrary, since a low efficient winding is made using the expansive dedicated winding machine, in the case of an integrated stator core structure, a manufacturing cost for the motors becomes high.
In order to employ the advantages of the axial gap type double rotor motor and the radial gap type core motor and improve the disadvantages thereof, a radial gap type core type double rotor structure BLDC motor has been proposed in Korean Patent Application Publication No. 10-432954 to the same applicant.
In the Korean Patent Application Publication No. 10-432954, rotors including respective permanent magnets are disposed in both the inner and outer sides of to thereby form flow of a magnetic path by the permanent magnets and the rotor yoke. It is thus possible to split the stator core completely into a plurality of stator core portions. Accordingly, productivity of the stator core can be heightened and a core material loss can be reduced through an individual coil winding process by using an inexpensive general purpose winding machine, and power of the motor can be greatly heightened by a combination of the stator core with a double-rotor.
In addition, in Korean Patent Application Publication No. 10-2005-245, there is provided an integral-type stator core that is manufactured by a process comprising: preparing a number of split-type core assemblies on which coils are wound; arranging the number of split-type core assemblies on which coils are wound on a printed circuit board PCB, so as to be fixed thereon; connecting the coils; and molding the number of split-type core assemblies arranged on a printed circuit board (PCB) in an annular shape by an insert molding with a thermosetting resin. Moreover, in the Korean Patent Application Publication No. 10-2005-245, there is proposed a stator structure in which split cores on which stator coils are wound are arranged alternately per phase of U, V, and W.
Meanwhile, a general BLDC motor uses a stator structure in which windings of the stator coils are arranged alternately in sequence per phase of U, V, and W, in the case of an integral type core, in which current flowing in the stator coils is selected alternately per phase according to a sequential switching drive of a switching transistor provided in an inverter circuit, to thereby generate a rotating magnetic field and to thus rotate a rotor.
For example, in the Korean Patent Application Publication Nos. 10-2005-245 and 10-2010-73449, there are provided motors of a double-rotor/single-stator structure each having a stator in which a number of split-type cores on which coils are wound are arranged alternately per phase of U, V, and W.
Hereinbelow, a method of designing a motor hereinafter, referred to as a “single-coil wiring structure” will be described in which one split core on which coils are wound per phase is disposed alternately per phase in a BLDC motor having a split core stator.
First, when designing a motor of a single-coil wiring structure typically, a setting between slots of a stator and rotor magnets magnetic poles is set as Equation 1.The number of poles=the number of slots/3×Multiple of 2  Equation 1
However, in the case that the number of slots is greater than, for example, 27 in designing a motor, the number of magnetic poles is set to be larger than the number of slots. As a result, according to Equation 1, the number of slots and poles of the motor, is set at a ratio of, for example, 18-slot and 12-pole, 27-slot and 36-pole, and 36-slot and 48-pole.
Hereinbelow, referring to FIGS. 1A to 3, a BLDC motor that is designed according to a single-coil wiring method of a double-rotor/single-stator structure using a split-core type stator in which conventional split cores on which coils are wound are arranged alternately per phase, will be described.
FIG. 1A shows a structure of a BLDC motor employing a double rotor structure that is designed with an 18-slot and 12-pole type that is designed according to a conventional single-coil wiring method. FIG. 1B shows a structure of a BLDC motor employing a double rotor structure having a shape of cores and magnets deformed to reduce cogging noise. FIG. 2 shows a coil wiring diagram and a motor drive circuit of drive coils of three phases U, V, and W that are applied in FIG. 1A. FIG. 3 is a diagram showing a relationship between a placement order and mutual wirings at the time of assembling split cores.
Referring to FIGS. 1A to 3, in the case that a BLDC motor 10 of a double rotor structure designed according to a conventional single-coil wiring method is of an 18-slot and 12-pole type, inner rotors 3a and outer rotors 3b are arranged at intervals, in which N-pole and S-pole magnets are alternately placed at the inside and outside of stators 1. The 18 split cores u11-u16, v11-v16, and w11-w16 on which coils u1-u6, v1-v6, and w1-w6 are respectively wound are disposed in an annular form.
Six stator coils u1-u6, v1-v6, and w1-w6 are mutually connected per phase U, V, or W of the stators 1, in which one side of each phase is connected to an output of U, V, or W of an inverter circuit 5 forming a motor drive circuit as shown in FIG. 2, and the other side of each phase is connected to each other to thus form a neutral point NP.
All of the stator coils u1-u6, v1-v6, and w1-w6 are wound on the split cores u11-u16, v11-v16, and w11-w16, respectively, in the forward direction, and are arranged alternately per phase U, V, or W and are assembled as shown in FIG. 3. In other words, the 18 split cores u11-u16, v11-v16, and w11-w16 are assembled in sequence of u1, v1, w1, u2, . . . , w5, u6, v6, and w6 and integrated on a stator support by an insert molding method using resins and formed in an annular form.
Hereinbelow, an operation of the BLDC motor 10 of a double rotor structure designed according to a conventional single-coil wiring method will be described with reference to Table 1.
TABLE 1Electrical0°60°120°180°240°300°360°, 0°angleMechanical0°10° 20° 30° 40° 50° 60°, 0°angleH1NNSSSNNH2SNNNSSSH3SSSNNNSInputUVVWWUUOutputWWUUVVWUpper FETFET1FET3FET3FET5FET5FET1FET1Lower FETFET2FET2FET4FET4FET6FET6FET2
As shown in Table 1, according to the conventional art, three Hall elements H1-H3 are sequentially placed between the slots, and are disposed at an angle that is determined by (360/the number of slots), that is, at intervals of 20°, to thereby detect a magnetic pole (N-pole or S-pole) of rotors 3a and 3b for each step and to then transmit the detected magnetic pole to a motor drive circuit.
The motor shown in FIG. 1A shows the status at 0° in which the directions of currents flowing in the stator coils u1-u6, v1-v6, and w1-w6 are changed per 10° in a six-step mode, and the currents whose flowing directions have been changed are applied to the corresponding split cores u11-u16, v11-v16, and w11-w16 to thereby form an electromagnet and to thus generate a magnetic field.
A motor drive circuit includes a controller (not shown) and an inverter circuit 5. The inverter circuit 5 is configured to include three pairs of switching transistors FETs that are connected in a totem pole structure, respectively, in which an output of each phase U, V, or W occurs from a junction between each of the upper switching transistors FETs that are FET1, FET3, and FET5 and each of the lower switching transistors FETs that are FET4, FETE, and FET2 corresponding to the upper switching transistors FETs that are FET1, FET3, and FET5, respectively, to thus be applied to the stator coils u1-u6, v1-v6, and w1-w6 of the motor 10. The controller (not shown) of the motor drive circuit controls the inverter circuit 5 to turn on a pair of the switching transistors FETs according to Table 1 to set a current flow path, if the position signals of the rotors 3a and 3b are detected by the Hall elements H1-H3 at the respective angles.
For example, as shown in FIG. 1A, the Hall elements H1-H3 detect the polarity of the outer rotor 3b as “N, S, S”, the controller (not shown) judges that the rotational position of the rotor is regarded as 0°, and applies drive signals to turn on the upper switching transistor FET1 and the lower switching transistor FET2. Accordingly, a current flows to ground via FET1, U-phase coils (u1-u6), W-phase coils (w6-w1), and FET2.
Accordingly, magnetic flux is generated in an inner direction of the split core u11, and magnetic flux is generated in an outer direction of the split core w11. Thus, a magnetic circuit is set as indicated by arrows of FIG. 1B, and the double rotor 3 is rotated clockwise in which the internal rotor 3a and the external rotor 3b are set as the N-pole and S-pole magnets to face each other.
In the BLDC motor 10 of FIG. 1A, for example, the split core u11 that generates the magnetic flux in the inner direction thereof is preferably configured so that an outside of the split core 11 is opposed to only the N-pole magnet 13 of the outer rotor 3b, but is also opposed to the S-pole magnet 14 adjacent to a part of the split core u11, and an inside of the split core 11 is opposed to only the S-pole magnet 13a of the inner rotor 3a, but is also opposed to the N-pole magnet 14a adjacent to a part of the split core u11, to thereby drop efficiency.
In this case, the split cores u11-u16, v11-v16, and w11-w16 of the stator 1 in the BLDC motor 10 are activated when drive signals are applied to coils of split cores while skipping a split core corresponding to one phase of three phases U, V, and W per step.
For example, as shown in FIG. 1A, when the rotational position of the rotor 3 is 0°, the split cores u11-u16 and w11-w16 of the stator 1 are activated when drive signals are applied to coils u1-u6 and w1-w6 wound on two consecutive U-phase and W-phase split cores u11-u16 and w11-w16 while skipping the V-phase split core v11-v16. As a result, the two consecutive split cores that have been activated generate the magnetic flux in opposite directions.
As described above, the BLDC motor 10 is configured so that the outside of the split core 11 is opposed to the S-pole magnet 14 adjacent to the N-pole magnet 13, in addition to the N-pole magnet 13, and the inside of the split core 11 is opposed to the N-pole magnet 14a adjacent to the S-pole magnet 14, in addition to the S-pole magnet 14, and a non-activated the split core v11 is disposed between the S-pole magnet 14 and the N-pole magnet 14a that face each other, to thus fail to form a magnetic circuit that is effective to rotate the rotor 3 in one direction.
In addition, in the case of the rotor 3 in the motor 10 shown in FIG. 1A, an offset is not be made at a portion where adjacent N-pole and S pole magnets overlap each other as the number of slots becomes smaller, to thereby cause the noise to occur.
Moreover, the BLDC motor 10 designed according to a conventional single-coil wiring method may be configured to set the slots and poles of the motor to have a structure of 18-slot and 12-pole, 27-slot and 36-pole, or 36-slot and 48-pole. In this case, since the ratio of the number of slots and poles becomes 30˜40% or so, an effective area of a magnetic force (or magnetic flux) between each of magnets 13-16 and each split core varies depending on the angle of rotation during rotation of the rotor 3. As a result, cogging may severely occur and magnetic flux leakage may occur.
Accordingly, as shown in FIG. 1B, to minimize noise caused by cogging, the BLDC motor 10 according to the conventional single-coil wiring method is required to have a wide opening width between a slot and another slot, to round an outer circumferential surface of each split core, and to edge-process (or round) corners of each magnet 13-16. As a result, a rise in production costs, and performance degradation may be caused.
After all, an opening width between a slot and another slot in the BLDC motor 10 according to the conventional single-coil wiring method should be wide to obtain good efficiency and to reduce noise, in which the opening width should be above a certain level. In addition, since the BLDC motor 10 according to the conventional single-coil wiring method is configured so that coils wound on the respective split cores may be connected together, a lot of wiring portions are also pointed out as disadvantages.
Moreover, as described above, since the BLDC motor 10 according to the conventional single-coil wiring method is configured so that the Hall elements H1-H3 may be sequentially placed at angles determined depending on (360°/the number of slots) or {(360°/the number of poles)×2 poles÷3}, a Hall element printed circuit board (PCB) on which three Hall elements are mounted should be disadvantageously made in size to cover a range of a 40° angle.
Meanwhile, since the Korean Patent Application Publication No. 10-2005-245 has a problem that an expensive large printed circuit board (PCB) should be used for wiring and assembly of coils with respect to a large number of split core assemblies, Korean Patent Registration No. 10-663641 was proposed in order to exclude a large printed circuit board (PCB) for assembly and to solve the wiring problems.
To this end, the Korean Patent Registration No. 10-663641 proposed a stator in which coils are continuously forwardly wound on nine split cores that are allocated for each phase of U, V, and W, the nine split cores are classified into three core groups, and the three core groups that are connected with three coils per phase are alternately arranged in turn.
In this case, each core group that has been connected with three coils is made for the purpose of removing the PCB for assembly and simultaneously solving the wiring problems between the respective split cores, in which coils are forwardly wound on the three split cores that have been connected with three coils.
As a result, since the motor of the three-coil wiring structure of the 27-slot and 24-pole structure is configured to have the three consecutive split cores on which coils are forwardly wound, a split core that is located in the middle of the three-coil wired three split cores offsets the magnetic flux of the other split cores that are disposed in the front and rear ends of the middle split core, to accordingly fail to contribute effectively in rotating the rotor and to improve the efficiency of the motor.
Meanwhile, a motor with a stator in which coils of a two-coil wiring method are sequentially arranged per phase has been proposed and the two-coil wiring method employs a forward and reverse wiring sequence of coils wound on teeth of an integral type stator core.
The motor employing the two-coil wiring method causes cogging noise smaller than the motor employing the single-coil wiring method, but causes a problem of increasing the number of coil wiring portions in comparison with the motor employing the three-coil wiring method, and a problem of enlarging a printed circuit board (PCB) for assembling Hall elements therewith since the Hall elements are disposed per two slots (30°). In addition, since a nozzle of a winding machine should enter between slots in an integral type stator core, an opening width between cores should be maintained over a certain range to thus enable windings. Thus, the motor employing the two-coil wiring method was applied for the purpose of reducing cogging not for the purpose of increasing efficiency of the motor.
As described above, the motor employing the conventional single-coil or two-coil wiring method has problems such as the high cogging noise, the low efficiency, a lot of coil wiring portions, and the large-sized PCB for assembling the Hall element therewith, in common